As methods for removing airborne hazardous substances of microbe origin such as viruses and bacteria, there are methods of filtration using various kinds of filters, physical adsorption using adsorbents, and the like.
The removal efficiency of airborne hazardous substances by means of a filter is greatly affected by the diameter and the density of a fiber used for the filter, the filter thickness, and the like. Typically, the removal efficiency increases as the fiber diameter of the filter becomes finer, the fiber density increases, or the filter thickness increases.
In order to improve the filtration precision and the filtration life of filters, there is proposed a filter comprising a superfine fiber assembly layer consisting of a fiber having an average fiber diameter of 0.01 μm or more but less than 0.5 μm, and a fine fiber assembly layer consisting of a fiber having an average fiber diameter of 0.5 μm or more but 5 μm or less (for example, JP Patent Publication (Kokai) No. 2005-218909). The use of such a fine fiber improves the filtration precision. However, if hazardous substances to be removed are bacteria or viruses, there is a likelihood in which the bacteria that have once captured by the filter are detached therefrom and reactivated to influence the human body.
In order to capture and inactivate bacteria or viruses by a filter, it is proposed to form a filter by using a fiber which has antibodies supported thereon with an official moisture regain of 7% or more (for example, JP Patent No. 3642340). Captured bacteria and the like are inactivated by the antigen-antibody reaction, by which the captured bacteria and the like can be kept from influencing the human body even if they are detached from the filter.
However, in order to maintain the activity of antibodies, the humidity environment in the vicinity of the fiber has to be indispensably controlled, and a high hygroscopic material such as a cellulosic fiber has to be contained therein. If the content of the cellulosic fiber is high, the strength of the fiber itself can not be kept, causing a need of enlarging the fiber diameter, which results in lowering of the removal efficiency.
From the above situation, there has been a demand for the development of fiber materials for filters which sufficiently satisfy required performances such as the high removal efficiency and mechanical properties, and enable to capture and inactivate bacteria, viruses, and the like.